Induction of Hemogenic Endothelium from Pluripotent Stem Cells by Forced Expression of Transcription Factors

ABSTRACT

Described herein are methods and related compositions for inducing differentiation of human pluripotent stem cells (hPSCs) into hemogenic endothelium with pan-myeloid potential or restricted potential, by forced expression in the hPSCs of a combination of transcription factors as described herein.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/716875 filed Oct. 22, 2012,which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under HL099773 awardedby the National Institutes of Health. The government has certain rightsin the invention.

BACKGROUND

Human pluripotent stem cells (hPSCs), including embryonic stem cells(hESCs) and induced PSCs (hiPSCs) offer a potentially plentiful sourceof blood cells for experimentation and therapeutic purposes. Significantadvances have been made in hematopoietic differentiation from hPSCsbased on the use of specific culture conditions designed to mimicdevelopmental processes. However, the identification of keytranscriptional regulators of hematopoietic commitment, and theiroverexpression would enable the directed and scaled conversion of humanpluripotent stem cells to hematopoietic stem cells (HSCs) and relatedblood cells.

SUMMARY

Described herein are methods and related compositions for generatinghuman hemogenic endothelial cells with pan-myeloid potential by forcedexpression, in human pluripotent stem cells, of combinations oftranscription factors as disclosed herein. Also described are methodsand related compositions for generating human endothelial cells withrestricted erythroid, megakaryotic, and macrophage potential by forcedexpression, in human pluripotent stem cells, of combinations oftranscription factors as disclosed herein.

Accordingly, in a first aspect provided herein is a method forgenerating human hemogenic endothelial cells with pan-myeloid potential,comprising the steps of: (i) forcing expression, in human pluripotentstem cells, of one of the following protein combinations: (a) an ETV2 orERG protein and a GATA1 protein, or functional homologs thereof; (b) anETV2 or ERG protein and a GATA2 protein, or functional homologs thereof;or (c) an ETV2 or ERG protein and a GFI 1 protein, or functionalhomologs thereof; and step (ii) culturing the human pluripotent stemcells following step (i), under culture conditions that supportexpansion of hematopoietic cells, to obtain hemogenic endothelial cellsthat are VE-cadherin⁺, CD226⁺, and CD73⁻.

In some embodiments, the method further comprises culturing thehemogenic endothelial cells of step (ii) for an additional period of atleast one to about four days to obtain CD43⁺ hematopoietic cells.

In some embodiments the culturing conditions of step (ii) includeculturing in the presence of FGF2, SCF, and thrombopoietin.

In some embodiments the forced expression lasts at least two to aboutthree days.

In some embodiments the forced expression in step (i) includestransduction of human pluripotent stem cells with a recombinantexpression virus, transfection with a double-stranded DNA expressionvector; transfection with a modified mRNA; protein transduction; or acombination thereof.

In some embodiments the ETV2 protein, ERG protein, GATA1 protein, orGATA2 protein, or GFI1 protein are from human, mouse, or rat.

In some embodiments the functional homologs are polypeptides selectedfrom the group consisting of: (i) a polypeptide comprising an amino acidsequence at least 90% identical to an ETV2, GATA1, ERG, GFI1, or GATA2protein from human, mouse, or rat, wherein the polypeptidetransactivates one or more target genes transactivated by the ETV2,GATA1, or GATA2 proteins in human pluripotent stem cells; and (ii) afusion polypeptide comprising an amino acid sequence that is at least90% identical to the amino acid sequence of of (a) a DNA binding domainof human, mouse, or rat ETV2, GATA1, ERG, GFI1, or GATA2 and (b) aheterologous transactivator domain; wherein the fusion polypeptidetransactivates one or more target genes transactivated by human, mouse,or rat ETV2, GATA1, ERG, GFI1, or GATA2 proteins in human pluripotentstem cells. In some embodiments the functional homologs comprise theamino acid sequence of human, mouse, or rat ETV2, GATA1, ERG, GFI1, orGATA2.

In another aspect disclosed herein is a method for generating humanhemogenic endothelial cells with restricted erythroid, megakaryocytic,and macrophage potential. The method includes the steps of: (i) forcingexpression, in human pluripotent stem cells, of a TAL1 protein and aGATA2 protein or GATA1 protein or functional homologs thereof; and (ii)culturing the human pluripotent stem cells following step (i), underculture conditions that support expansion of hematopoietic cells, toobtain hemogenic endothelial cells that are VE-cadherin⁺, CD226⁺, CD73⁻,and have restricted erythroid, megakaryocytic, and macrophage potential,wherein the forced expression.

In some embodiments, the forced expression in this method does notinclude forced expression of LMO2.

In some embodiments the above method further includes culturing thehemogenic endothelial cells (with restricted erythroid, megakaryocytic,and macrophage potential) for an additional period to obtainerythrocytes, megakaryocytes, or macrophages.

In some embodiments the method further comprises forcing the expressionof LMO2 or a functional homolog thereof in the human pluripotent stemcells.

In a further aspect described herein is a recombinant human pluripotentstem cell comprising: (i) one or more exogenous nucleic acids suitablefor expression of (a) an ETV2 or ERG protein, and a GATA1 protein, orfunctional homologs thereof; (b) an ETV2 or ERG protein, and a GATA2protein, or functional homologs thereof; or (c) an ETV2 or ERG protein,and a GFI1 protein (ii) exogenous polypeptides comprising the amino acidsequences of any of (a), (b), or (c).

In some embodiments the exogenous polypeptides comprise the amino acidsequence of a protein transduction domain.

In some embodiments the recombinant human pluripotent stem cell isintegration free. In some embodiments, exogenous nucleic acids in theintegration-free human pluripotent stem cell are episomal expressionvectors. In other embodiments, the exogenous nucleic acids in theintegration-free recombinant human pluripotent stem cells are modifiedmRNAs.

In a related aspect described herein is a cell culture composition forgenerating human hemogenic endothelial cells with pan-myeloid potential,comprising any of the above-mentioned recombinant human pluripotent stemcells and a cell culture medium suitable for expansion of hematopoieticcells. In some embodiments the suitable cell culture medium includesFGF2, SCF, and thrombopoietin.

In yet another aspect described herein is a kit for hemogenicreprogramming that includes: (i) one or more isolated nucleic acidscomprising an open reading frame for (a) ETV2 or ERG, and GATA1; (b)ETV2 or ERG, and GATA2; (c) ETV2 or ERG, and GFI1; or (c) TAL1 andGATA2; or (ii) one or more recombinant expression viruses suitable forexpression, in human pluripotent stem cells, of (a) ETV2 or ERG, andGATA1; (b) ETV2 or ERG, and GATA2; (c) ETV2 or ERG, and GFI1; or (c)TAL1 and GATA2.

In some embodiments the one or more isolated nucleic acids in the kitare modified mRNAs.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Gain-of-function screening in hPSCs. (a) Schematic diagram ofscreening system; (b-d) Expression of pluripotency markers in H1 hESCsgrowing on Matrigel™ for 5 days in standard conditions TeSR1™ medium (b)and TeSR1™ medium containing SCF (100 ng/ml) TPO (50 ng/ml) and bFGF (20ng/ml) (c and d); (e) Flow cytometric analysis of mesodermal,endothelial and hematopoietic markers in control hESCs and hESCstransduced with indicated TFs on day 5 post-transduction; (f-g) ETV2-and ERG-transduced cells acquire endothelial characteristics as shown bypositive VE-cadherin immunostaining, AcLDL uptake (f) and formation ofendothelial tubes in solidified Matrigel™ in the presence of VEGF (g).Scale bar, d,f,g, 100 μm.

FIG. 2 Gene expression profiling of H1 hESCs differentiated byoverexpression of single transcription factor and blood inducingcombinations. (a) Heat map of selected sets of genes associated withendothelial and hematopoietic differentiation. (b) Heat map of selectedset of genes associated with the development of germ layers and theirderivatives.

FIG. 3 Hematopoietic differentiation of hES cells induced byco-expression of GATA2/ETV2 and GATA2/TAL1 (LMO2). (a) Cell morphologyand flow cytometric analysis of GATA2/ETV2-transduced H1 hESCs on day 7post-transduction. Scale bar, 100 μm. (b) Types of hematopoieticcolonies formed by GATA2/ETV2-transduced cells. Erythroid colonies(CFC-E); macrophage colonies (CFC-M); High Proliferative Potential(CFC-HPP) myeloid colonies containing predominantly myeloblasts withsome granulocytes and macrophages. Scale bar for CFC-assay, 250 μm;cytospins, 20 μm. (c) Phase-contrast photograph of the culture,Wright-stained cytospin and FACS analysis of GATA2/ETV2-inducedhematopoietic cells grown in low attachment culture for 14 dayssupplemented with 30% FBS and hematopoietic cytokines (SCF-100 ng/ml,IL3-10 ng/ml, IL6-20 ng/ml, GM-CSF-10 ng/ml, G-CSF-20 ng/ml, EPO -3u/ml). (d) CFC potential of cells transduced with ETV2 and indicated TFcombinations. Error bars represent SE from 2 to 5 independentexperiments. (e) Cell morphology and flow cytometric analysis of H1hESCs differentiated by expression of GATA2/TAL1 on day 7post-transduction. (f) Types of hematopoietic colonies formed byGATA2/TAL1- and GATA2/TAL1/LMO2-differentiated cells. Erythroid colonies(CFU-E); macrophage colonies (CFU-M); Megakaryocytic colonies (CFU-Mk).Scale bar for CFC-assay, 250 μm; Scale bar on cytospins, 20 μm. (g)Phenotypic characterization of GATA2/TAL1/LMO2-induced hematopoieticcells grown in serum-free culture supplemented with SCF (100 ng/ml), TPO(50 ng/ml) and bFGF (20 ng/ml) for 14 days. (h) CFC potential of hESCstransduced with GATA2-based combinations. Error bars represent SE from 3independent experiments.

FIG. 4 Direct hematopoietic programming of undifferentiated H1 hESCsgoes through an endothelial stage. (a) Kinetic analysis of VE-cadherinand CD43 expression during direct programming of H1 hESCs by GATA2/ETV2,GATA2/TAL1 and GATA2/TAL1/LMO2 TFs by flow cytometry. (b) VE-cadherinand CD43 immunofluorescent staining of untreated control hESCs and hESCstransduced with indicated TFs at different time points aftertransduction. Scale bars, 100 μm. (c) Expression of markers associatedwith hemogenic and non-hemogenic endothelium by VE-cadherin+ cellsemerging on day 3 post-transduction with indicated TFs.

FIG. 5 Hematopoietic induction of hESCs with ETV2/GATA2 mmRNA. (a) Flowcytometric analysis and (b) CFC potential of mmRNA-induced cells.

FIG. 6 Design of screening system. (a) TFs enriched in hESC-derivedmesodermal and endothelial cells with hematopoietic activity (MaximVodyanik et al., 2010; Kung-Dal Choi et al., 2012). Bars represent aratio of TF expression in indicated subpopulations obtained from hESCsdifferentiated in coculture with OP9 and analyzed by RNAseq. HE isVE-cadherin⁺CD43⁻CD73⁻ hemogenic endothelium, non-HE(VE-cadherin⁺CD43⁻CD73⁺) non-hemogenic endothelium, PM is apelinreceptor positive primitive mesodermal cells with hemangioblastpotential generated on day 3 hESC/OP9 coculture, HVMP is hematovascularmesodermal precursor highly enriched in cells forming hematoendothelialclusters on OP9 isolated on day 4 of differentiation, HB is endothelialintermediates (cores) with hematopoietic activity formed inhemangioblast clonogenic cultures, MB is endothelial intermediates(cores) without hematopoietic activity formed in mesenchymoangioblastclonogenic cultures. (b) Phase-contrast and fluorescent microscopy of H1hESC transduced with eGFP-expressing virus, day 5 post-transduction(0.68×10⁶ cells, MOI=0.5). Right panel shows efficiency of lenti-viraltransduction in hESCs by FACS analysis. (c) PCR analysis of virusintegration into genome. 10⁴ H1 cells transduced with indicatedconstructs were collected for DNA isolation, followed by pSIN-EFaspecific PCR. (d) RT-PCR of indicated transgenes in HeLa cellstransduced with indicated constructs. (e) Western blot analysis showsoverexpressed proteins in HeLa cells transduced with correspondingpSIN-EFla expression vectors: SCL-FLAG, LMO2-FLAG, LYL1-FLAG, HHEX-HA,GATA2-HA, FLI1-myc, MYB-myc; (f, g) Real-time PCR analysis of transgeneand endogenous expression of ETV2 and GATA2 transcripts in hematopoieticcolonies derived from ETV2 and GATA2 transduced cells on day 21post-transduction.

FIG. 7 Morphologies of hESCs differentiated by the overexpression ofsingle transcription factors. Phase-contrast microscopy of H1 hESCscultures transduced with indicated transcription factors. Photographswere taken at the day of cell collection as indicated in Table 4. Scalebar, 100 μm.

FIG. 8 Screening of different combinations of TFs based on co-expressionwith ETV2 and ERG. (a) CD43 expression by hESCs on day 7 aftertransduction with ETV2 plus indicated TFs. (b,c) Advanced hematopoieticprogramming of cells requires combination of ETV2 with GATA2 but notTAL1 and LMO2 as reflected in the amount of CD43 positive cells on day 7post-transduction (b), and corresponding colony forming activity (c).(d) Comparative analysis of programming combinations with multiple TFsbased on GATA2/ETV2 co-expression. (e) Hematopoietic programming withERG-based combinations assessed by CFU-assay. Error bars in (c) and (e)show SE from two to four independent experiments. (d) shows results fromrepresentative experiment.

FIG. 9 Hematopoietic potential of VE-cadherin⁺CD43⁻CD73⁻ endothelialcells isolated from programming cultures. (a) On day 3 aftertransduction with GATA2/ETV2 and GATA2/TAL1, VE-cadherin⁺CD43⁻CD73⁻cells were isolated by sorting and cultured on OP9 to assesshematopoietic potential. (b) Quantification of clonal hematopoieticclusters developed from VE-cadherin⁺CD43⁻CD73⁻ cells on OP9. Error barrepresents 3 independent experiments. (c) Immunofluorescent staining ofhematopoietic clusters developed from single VE-cadherin⁺ CD43⁻CD73⁻cells deposited on OP9 monolayer. Scale bar, 100 μm.

FIG. 10 Screening of different combinations of TFs based on GATA2 andTAL1. (a, b, c) FACS analysis of total cultures collected on day 7 aftertransduction of hESCs with indicated TFs. (d) Analysis of CFC potentialof cells co-expressing erythroid factors (GATA2, TAL1, LMO2) and myeloidfactors (SPI1, GFI1, MYB, FLI1, RUNX1C/B). (e) morphology and flowcytometric analysis of colonies formed by hESCs transduced with SPI1,GFI1, MYB, FLI1, RUNX1C/B

FIG. 11 Hematopoietic programming of 119 hESCs and iPSCs. (a) FACSanalysis of TFs differentiated iPS cell lines DF-19-9-7T and DF-4-3-7Tgrown in low attachment conditions in the presence of cytokines and 10%FBS. (b) Colony forming units developed from H9 hESCs by induction ofGATA2/ETV2 and GATA2/TAL1/LMO2.

DETAILED DESCRIPTION

Advancing pluripotent stem cell technologies for modeling HSCdevelopment and therapies requires identification of the key regulatorof hematopoietic commitment from human pluripotent stem cells (hPSCs).Transcription factors (TFs) have been recognized as critical controllersof early embryonic development. The factors are thought to function askey elements of the gene regulatory network that guide the acquisitionof specific properties by particular cell type. To define the key TFsrequired for induction of blood, we performed gain-of-function geneticscreens in human embryonic stem cells (hESCs) to identify specificcombinations of TFs that induced differentiation of hPSCs into humanhemogenic endothelial cells with pan-myeloid potential. In some casesthe identified combination of transcription factors induceddifferentiation of hPSCs into hemogenic endothelial cells withrestricted erythroid, megakaryocytic, and macrophage potential. Theidentified transcription factors are referred to as “induction factors,”(IFs) herein.

I. Definitions

“Forced Expression” refers to inducing an increase in the level of aprotein of interest (e.g., a transcription factor) in a population ofhost cells, e.g., hPSCs. Forced expression can include one or more ofthe following in any combination: introducing exogenous nucleic acidsencoding the protein of interest (e.g., by viral transduction, plasmidexpression vector transfection, or modified mRNA transfection); proteintransduction; genomic modification of a host cell, e.g., replacing apromoter to increase the expression of an endogenous (native) gene; andcontacting host cells with a small molecule that induces increasedexpression of an endogenous protein.

“Functional Homolog” refers to an induction factor that transactivatesat least some of the same promoters or target genes as the referenceinduction factor. In some cases, the functional homolog transactivates acognate promoter of induction factor with at least 10% to 95% of thecorresponding activity, e.g., 15%, 20%, 30%, 40%, 50%, 60%, 70%, 80%,90%, or another level of transactivation activity of the referenceinduction factor from at least 10% to 95%. A functional homolog can alsobe a paralog, i.e., a naturally occurring protein having at least 80% to99% amino acid sequence identity to the reference induction factor,similar function, and belonging to the same protein family.

“Integration-Free” refers to the absence of exogenous sequences in agenome;

“Induction Factor” refers to a protein the forced expression of whichdrives differentiation of host cells (e.g., hPSCs) into hemogenicendothelial cells.

“Recombinant Expression Virus” refers to a virus comprising a proteincapsid and a genome that includes an expression cassette suitable forexpression in a mammalian host cell.

“Recombinant Human Pluripotent Stem Cell” refers to an hPSC (e.g., anhiPSC or hESC) that comprises either an exogenous nucleic acid encodinga polypeptide (e.g., an expression vector or a modified mRNA), or anexogenous polypeptide.

II. Methods

Generation of Hemogenic Endothelial Cells

In some embodiments described herein is a method for generating humanhemogenic endothelial cells with pan-myeloid potential, comprisingforcing expression in hPSCs of one of the combinations of IFs describedbelow; and, afterwards, culturing the human pluripotent stem cells underculture conditions that support expansion of hematopoietic cells toobtain hemogenic endothelial cells that are VE-cadherin⁺, CD226⁺, andCD73⁻.

Prior to forced expression hPSCs are grown by any of a number of knownmethods in the art, although, preferably, the hPSCs are grown for atleast two passages, prior to forced expression, under feeder-freeconditions, e.g., in TeSR™ or E8™ culture medium in combination with anextracellular matrix substrate such as Matrigel™ or vitronectin.

Forced expression of a combination of IFs, or functional homologsthereof, is then carried out by any of a number of methods describedherein to obtain human hemogenic endothelial cells with pan-myeloidpotential, or, in other embodiments, to obtain human hemogenicendothelial cells with restricted potential.

After initiating forced expression of IFs in the hPSCs, these cells arecultured for a period of about 24 hours in a medium suitable for cultureof hPSCs, e.g., complete TeSR1™ medium on an extracellular matrixsubstrate (e.g., Matrigel™), after which the medium is replaced withgrowth factor-free TeSR1™ base medium supplemented with stem cell factor(SCF; 10-200 ng/ml); thrombopoietin (TPO; 10-200 ng/ml) and FGF2 (10-100ng/ml), and the cells are cultured for up to seven days, after whichcells can be cultured in complete StemSpan™ SFEM medium (StemCellTechnologies, Vancouver) or StemLine® HSC medium prior to identificationof differentiated cells. In some embodiments, the forced expressionculture period is for at least two to about three days. In someembodiments the forced expression culture period is for about two toabout seven days prior to analysis of differentiation.

The resulting human hemogenic endothelial cells are identified asVE-cadherin⁺, CD226⁺, and CD73⁻ cells. In some embodiments, thehemogenic endothelial cells are isolated by cell sorting to initiateclonogenic cultures in the presence of OP9 stromal cells, to generatecolonies of CD43⁺ hematopoietic cells with multilineage colony formingcell (CFC) potential. In some embodiments, the CD43⁺ cells generatedfrom hemogenic endothelium subjected to colony-forming assay inserum-containing methylcellulose medium (e.g., MethoCult™, Stem CellTechnologies) supplemented with SCF, G-CSF, GM-CSF, IL3, IL6, and EPO.In some embodiments, colony-forming cells expanded in culturescontaining TeSR1™ or aMEM medium with 30% FBS and hematopoieticcytokines (SCF-100 ng/ml, IL3-10 ng/ml, IL6-20 ng/ml, GM-CSF-10 ng/ml,G-CSF-20 ng/ml, EPO −3 u/ml) or SFEM medium supplemented with SCF-100ng/ml, TPO 50 ng/ml and FGF2 20 ng/ml. The expansion cultures can thenbe assessed for various types of myeloid cells. Cultures withpan-myeloid potential give rise to CD34⁺CD117⁺ primitive progenitors,CD163⁺ macrophages, CD66b⁺ granulocytes, CD41a⁺ megakaryocytic andCD235a⁺ erythroid cells. Cultures with restricted myeloid potential,generated as described herein, generate almost exclusively CD235a⁺erythroid and CD41a⁺ megakaryocytic cells. Cell surface characterizationof, or isolation of cells obtained by the above-described methods can beperformed by a number of methods known in the art including, but notlimited to, flow cytometry, magnetic-activated cell sorting (MACS), andacoustic cell sorting.

Induction Factors

Suitable combinations of IFs to obtain human hemogenic endothelial cellswith pan-myeloid potential include any of those listed in Table 1:

TABLE 1 IF Combinations to Induce Human Hemogenic Endothelial Cells withPan-Myeloid Potential from hPSCs Combination IFs I ETV2 (SEQ ID NO: 1)and GATA1 (SEQ ID NO: 2) II ERG (SEQ ID NO: 3) and GATA1 (SEQ ID NO: 2)III ETV2 (SEQ ID NO: 1) and GATA2 (SEQ ID NO: 4) IV ERG (SEQ ID NO: 3)and GATA2 (SEQ ID NO: 4) V ETV2 (SEQ ID NO: 1) and GFI1 (SEQ ID NO: 5)VI ERG (SEQ ID NO: 3) and GFI1 (SEQ ID NO: 5)

Suitable combinations of IFs to obtain human hemogenic endothelial cellswith restricted erythroid, megakaryocytic, and macrophage potentialinclude any of those listed in Table 2:

TABLE 2 IF Combinations to Induce Human Hemogenic Endothelial Cells andBlood Cells with Restricted Potential from hPSCs Combination IFs I-RTAL1 (SEQ ID NO: 6) and GATA1 (SEQ ID NO: 2) II-R TAL1 (SEQ ID NO: 6)and GATA2 (SEQ ID NO: 4) III-R TAL1 (SEQ ID NO: 6) and GATA1 (SEQ ID NO:2) and LMO2 (SEQ ID NO: 7) IV-R TAL1 (SEQ ID NO: 6) and GATA2 (SEQ IDNO: 4) and LMO2 (SEQ ID NO: 7)

In some embodiments, the combinations of IFs used to inducedifferentiation of hPSCs into hemogenic endothelial cells withrestricted potential do not include LMO2. Preferably, the IFs listed inTables 1 and 2 correspond to the human homologs of these proteins,however, in other embodiments, one or more of the IFs are mouse or rathomologs.

In other embodiments one or more of the listed IFs to be used in themethods described herein are functional homologs of IFs listed in Tables1 and 2.

In some embodiments one or more of the IFs listed in Table 1 or Table 2is replaced with a functional homolog. A functional homolog, in the caseof a transcription factor, refers to a transcription factor thattransactivates at least some of the same promoters or target genes asthe reference IF. In some cases, the functional homolog transactivates acognate promoter of one the above-mentioned IFs with at least 10% to 95%of the corresponding activity, e.g., 15%, 20%, 30%, 40%, 50%, 60%, 70%,80%, 90%, or another level of transactivation activity of the referenceIF from at least 10% to 95%. Methods for measuring transactivationactivity are well known in the art and include, but are not limited to,promoter-reporter activity assays (e.g., promoter-luciferase assays) andthe like. In some embodiments, the functional homolog is a paralog,i.e., a naturally occurring protein having at least 80% to 99% aminoacid sequence identity (e.g., 85%, 90%, 92%, 94%, 95%, 97%, or anotherpercent identity) to the reference IF, similar function, and belongingto the same protein family.

In some embodiments an IF functional homolog is a polypeptide comprisingan amino acid sequence at least 90%, e.g., identical to an ETV2, ERG,GATA1, GATA2, GFI1, or TAL1 protein from human, mouse, or rat, where thepolypeptide transactivates one or more cognate target genes of theforegoing IFs.

In some embodiments, the DBD amino acid sequence of one of the IFs to beused is at least 85% to 100% identical to the DBD sequence of the DBDamino acid sequence of a mouse, rat, human, or chicken homolog of one ofthe IFs listed in Table 1 or Table 2, e.g., at least 90%, 92%, 93%, 95%,97%, 99%, or another percent amino acid identity from at least 85% to100% identical. In other embodiments, the functional homolog DBD,contains up to 10 amino acid changes (i.e., deletions, insertions, orsubstitutions), i.e., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 amino acidchanges in the amino acid sequence of the DBD.

The skilled artisan also recognizes that transcription factors oftencontain discrete DNA binding domains (DBDs) and transactivation domains(TDs), and in many cases it is possible to substitute a nativetransactivation domain with an unrelated transactivation domain (e.g.,VP16, GAL4, or LEX TDs) well known in the art and often used to generatefunctional heterologous transcription factors having a desired DBD,e.g., the GATA2 DBD, and a heterologous TD, e.g. GATA1(DBD)-VP16, asexemplified in Blobel et al (1995), Mol Cell Biol, 15(2):626-633. Theamino acid sequence of the VP16 transactivation domain (SEQ ID NO:13) isprovided below:

(SEQ ID NO: 13) TKTLMKKDKYTLPGGLLAPGGNSMASGVGVGAGLGAGVNQRMDSYAHMNGWSNGSYSMMQDQLGYPQHSTTAPITDVSLGDELRLDGEEVDMTPADALDDFDLEMLGDVESPSPGMTHDPVSYGALDVDDFEFEQMFTDALGIDDFG G

In yet other embodiments a functional homolog may comprise the aminoacid sequence of an artificial transcription factor that has nosignificant sequence identity to any of the reference IF sequences (SEQID NOs:1-7), but is able to bind and transactivate a cognate promotersequence. For example, the artificial DBD may be generated by designingzinc finger-containing proteins having binding specificity for adesigned target sequence (e.g., a GATA motif). The zinc-finger DBD isthen fused to a transactivator protein, e.g., VP16 to generate a fusionprotein that is an artificial TF. See, e.g., Wilson et al (2013), MolTher Nucleic Acids, (published online): 2, e87; doi:10.1038; and Klug(2010), Q Rev Biophys.; February; 43(1):1-21. doi:10.1017/S0033583510000089.

Evaluating the structural and functional homology of two or morepolypeptides generally includes determining the percent identity oftheir amino acid sequences to each other. Sequence identity between twoor more amino acid sequences is determined by conventional methods. See,for example, Altschul et al., (1997), Nucleic Acids Research,25(17):3389-3402; and Henikoff and Henikoff (1982), Proc. Natl. Acad.Sci. USA, 89:10915 (1992). Briefly, two amino acid sequences are alignedto optimize the alignment scores using a gap opening penalty of 10, agap extension penalty of 1, and the “BLOSUM62” scoring matrix ofHenikoff and Henikoff (ibid.). The percent identity is then calculatedas: ([Total number of identical matches]/[length of the shorter sequenceplus the number of gaps introduced into the longer sequence in order toalign the two sequences])(100).

Those skilled in the art will appreciate that there are many establishedalgorithms available to align two amino acid sequences. The “FASTA”similarity search algorithm of Pearson and Lipman is a suitable proteinalignment method for examining the level of identity shared by an aminoacid sequence disclosed herein and the amino acid sequence of anotherpeptide. The FASTA algorithm is described by Pearson and Lipman (1988),Proc. Nat'l Acad. Sci. USA, 85:2444, and by Pearson (1990), Meth.Enzymol., 183:63. Briefly, FASTA first characterizes sequence similarityby identifying regions shared by the query sequence (e.g., any of SEQ IDNOs:1-7) and a test sequence that have either the highest density ofidentities (if the ktup variable is 1) or pairs of identities (ifktup=2), without considering conservative amino acid substitutions,insertions, or deletions. The ten regions with the highest density ofidentities are then rescored by comparing the similarity of all pairedamino acids using an amino acid substitution matrix, and the ends of theregions are “trimmed” to include only those residues that contribute tothe highest score. If there are several regions with scores greater thanthe “cutoff' value (calculated by a predetermined formula based upon thelength of the sequence and the ktup value), then the trimmed initialregions are examined to determine whether the regions can be joined toform an approximate alignment with gaps. Finally, the highest scoringregions of the two amino acid sequences are aligned using a modificationof the Needleman-Wunsch-Sellers algorithm (Needleman and Wunsch (1970),J. Mol. Biol., 48:444-453; Sellers (1974), SIAM J. Appl. Math., 26:787),which allows for amino acid insertions and deletions. Illustrativeparameters for FASTA analysis are: ktup=1, gap opening penalty=10, gapextension penalty=1, and substitution matrix=BLOSUM62. These parameterscan be introduced into a FASTA program by modifying the scoring matrixfile (”SMATRIX″), as explained in Appendix 2 of Pearson (1990), Meth.Enzymol., 183:63.

A number of considerations are useful to the skilled artisan indetermining if a particular amino acid sequence variant of one of theIFs described herein is likely to have suitable transcriptional activitycompared to an IF comprising a naturally occurring IF reference aminoacid sequence (e.g., ETV2 (SEQ ID NO:1). These considerations include,but are not limited to: (1) known structure-function relationships forthe variant polypeptide, e.g., the presence of discrete functionaldomains, e.g., a DBD; and (2) the presence of amino acid sequenceconservation among naturally occurring homologs (e.g., in paralogs andorthologs) of an IF, as revealed by sequence alignment algorithms asdescribed herein. Notably, a number of bioinformatic algorithms areknown in the art that successfully predict the functional effect, i.e.,“tolerance” of particular amino substitutions in the amino acid sequenceof a protein on its function. Such algorithms include, e.g., pMUT, SIFT,PolyPhen, and SNPs3D. For a review see, e.g., Ng and Henikoff (2006),Ann Rev Genomics Hum Genet., 7:61-80. For example, pMUT predicts with ahigh degree of accuracy (about 84% overall) whether a particular aminoacid substitution at a given sequence position affects a protein'sfunction based on sequence homology. See Ferrer-Costa et al., (2005),Bioinformatics, 21(14):3176-3178; Ferrer-Costa et al., (2004), Proteins,57(4):811-819; and Ferrer-Costa et al., (2002), J Mol Biol, 315:771-786.The SIFT algorithm server is publicly available on the world wide webat: blocks.fhcrc.org/sift/SIFT.html.. Thus, for any IF functionalhomolog amino acid sequence, an “amino acid substitution matrix” can begenerated that provides the predicted neutrality or deleteriousness ofany given amino acid substitution on IF function.

In preferred embodiments, where an amino acid is to be substitutedwithin one of the IF reference sequences disclosed herein, the aminoacid substitution is a conservative amino acid substitution. Among thecommon amino acids, for example, a “conservative amino acidsubstitution” is illustrated by a substitution among amino acids withineach of the following groups: (1) glycine, alanine, valine, leucine, andisoleucine, (2) phenylalanine, tyrosine, and tryptophan, (3) serine andthreonine, (4) aspartate and glutamate, (5) glutamine and asparagine,and (6) lysine, arginine and histidine. The BLOSUM62 table is an aminoacid substitution matrix derived from about 2,000 local multiplealignments of protein sequence segments, representing highly conservedregions of more than 500 groups of related proteins (Henikoff andHenikoff, Proc. Nat'l Acad. Sci. USA 89:10915 (1992)). Accordingly, theBLOSUM62 substitution frequencies can be used to define conservativeamino acid substitutions that may be introduced into the amino acidsequences of the present invention. Although it is possible to designamino acid substitutions based solely upon chemical properties (asdiscussed above), the language “conservative amino acid substitution”preferably refers to a substitution represented by a BLOSUM62 value ofgreater than −1. For example, an amino acid substitution is conservativeif the substitution is characterized by a BLOSUM62 value of 0, 1, 2, or3. According to this system, preferred conservative amino acidsubstitutions are characterized by a BLOSUM62 value of at least 1 (e.g.,1, 2 or 3), while more preferred conservative amino acid substitutionsare characterized by a BLOSUM62 value of at least 2 (e.g., 2 or 3).

Non-naturally occurring sequence variants can be generated by a numberof known methods. Such methods include, but are not limited to, “GeneShuffling,” as described in U.S. Pat. No. 6,521,453; “RNA mutagenesis,”as described in Kopsidas et al., (2007), BMC Biotechnology, 7:18-29; and“error-prone PCR methods.” Error prone PCR methods can be divided into(a) methods that reduce the fidelity of the polymerase by unbalancingnucleotides concentrations and/or adding of chemical compounds such asmanganese chloride (see, e.g., Lin-Goerke et al., (1997), Biotechniques,23:409-412), (b) methods that employ nucleotide analogs (see, e.g., U.S.Pat. No. 6,153,745), (c) methods that utilize ‘mutagenic’ polymerases(see, e.g., Cline, J. and Hogrefe, H. H. (2000), Strategies (StratageneNewsletter), 13:157-161 and (d) combined methods (see, e.g., Xu et al.,(1999), Biotechniques, 27:1102-1108. Other PCR-based mutagenesis methodsinclude those, e.g., described by Osuna et al., (2004), Nucleic AcidsRes., 32(17):e136 and Wong et al., (2004), Nucleic Acids Res., 10;32(3):e26), and others known in the art.

In some embodiments, forced expression of IFs factors in hPSCs isachieved by any of a number of established methods to introduce amammalian expression vector, e.g., viral transduction, lipofection,electroporation, or nucleofection. In some embodiments mammalianexpression vectors to be used are double-stranded nucleic acid vectors(e.g., episomal plasmid vectors, transposon vectors, or minicirclevectors). Mammalian expression vectors suitable for the methodsdescribed herein comprise a promoter competent to drive IF expression inhPSCs. Examples of suitable promoters for driving IF expression in hPSCsinclude, but are not limited to, constitutive promoters such as, EF-1-α,CAG, Ubiquitin (UbC), cytomegalovirus (CMV), HSV1-TK, SV40,.β-actin;PGK, and inducible promoters, such as those containing TET-operatorelements.

In some embodiments, a mammalian expression vector used herein comprisesa polycistronic expression cassette, i.e., an expression cassette thatencodes a “polyprotein” comprising multiple polypeptide sequences thatare separated by encoded by a picornavirus, e.g., a foot-and-mouthdisease virus (FMDV) viral 2A peptide sequence. The 2A peptide sequenceacts co-translationally, by preventing the formation of a normal peptidebond between the conserved glycine and last proline, resulting inribosome skipping to the next codon, and the nascent peptide cleavingbetween the Gly and Pro. After cleavage, the short 2A peptide remainsfused to the C-terminus of the ‘upstream’ protein, while the proline isadded to the N-terminus of the ‘downstream’ protein, which duringtranslation allow cleavage of the nascent polypeptide sequence intoseparate polypeptides. See, e.g., Trichas et al (2008), BMC Biol, 6:40.

In other embodiments, a polycistronic expression cassette mayincorporate one or more internal ribosomal entry site (IRES) sequencesbetween open reading frames incorporated into the polycistronicexpression cassette. IRES sequences and their use are known in the artas exemplified in, e.g., Martinez-Salas (1999), Curr Opin Biotechnol,10(5):458-464.

In some embodiments forced expression of an IF is carried out bytransducing hPSCs with one or more recombinant expression virusescarrying DNA or RNA encoding one or more of the above-described IFs.Examples of recombinant viruses include, but are not limited to,retroviruses (including lentiviruses); adenoviruses; adeno-associatedviruses, Herpes Simplex virus (HSV), and RNA viruses such as Sendai(RNA) virus.

In one embodiment, forced expression of IFs in hPSCs is carried out byuse of recombinant lentiviruses comprising an EF-α promoter to driveexpression of bicistronic expression cassettes encoding an IF and linkedby way of an IRES sequence to a selection marker, e.g., a proteinencoding resistance to puromycin. Typically, where lentiviruses areused, hPSCs are transduced in single cell suspension in complete TeSR1™medium at a concentration of about 0.5×10⁶ to 1×10⁶ 6.8×10⁵ cells/ml inthe presence of a Rho kinase inhibitor, e.g., Y27632 (10 μg/ml) andpolybrene (6 μg/ml) at a multiplicity of infection (MOI) of about 1 to5. The hPSC and virus-containing medium suspension is then plated onMatrigel™-cell culture plates and incubated for about 12 hours, afterwhich the medium is replaced with fresh TeSR1™ medium, and the cellsincubated for another 12 hours prior to culturing in growth-factorsupplemented medium as described above.

In some embodiments double stranded DNA expression vectors (“DNAexpression vectors”) are used to express IFs as described herein. Insome embodiments, the DNA expression vectors used in the reprogrammingmethods described herein also include loxP transposition target sitesfor CRE recombinase, which allows for subsequent excision of the vector.In other embodiments DNA expression vectors are episomal vectors thatare stably maintained and replicate within host mammalian cells withoutgenomic integration. Episomal vectors include a mammalian origin ofreplication, e.g., the Epstein-Barr Virus oriP element (Yates et al(1984), Proc. Natl. Acad. Sci. USA, 81:3806-3810, which allows episomalreplication of the DNA expression vector in the hPSCs. Examples ofvectors comprising a mammalian origin of replication are described in,e.g., U.S. Pat. No. 8,546,140. Episomal DNA expression vectors suitablefor the methods described herein include, but are not limited to, any ofthe following episomal vectors: pCEP4, pREP4, or pEBNA DEST. In someembodiments, the DNA expression vectors suitable for the methodsdescribed herein include a S/MAR (scaffold/matrix attachment region)sequence. See, e.g., Piechaczek et al (1999), Nucleic Acids Res,27:426-428.

In some embodiments, the mammalian expression vectors to be used arepiggyBac transposon expression vectors, which are efficiently integratedinto the genome of mammalian cells when transfected into the mammaliancells in the presence of a piggyBac transposase. Subsequently, apiggyback transposon can be excised from the genome of recombinant hostcells, by transiently expressing a piggyback transposase. See, e.g.,Yusa et al (2011), Proc. Natl Acad. Sci USA, 108:1531-1536.

In some embodiments forced expression of IFs is achieved by introductionof modified mRNAs (mmRNAs) encoding IFs into hPSCs, e.g., byelectroporation. mmRNAs and their synthesis is described in detail in,e.g., U.S. Patent Application Publication No 20120046346. Typically,mmRNAs comprise (i) a 5′ synthetic cap for enhanced translation; (ii)modified nucleotides that confer RNAse resistance and an attenuatedcellular interferon response, which would otherwise greatly reducetranslational efficiency; and (iii) a 3′ poly-A tail. Typically, IFmmRNAs are synthesized in vitro from a DNA template comprising an SP6 orT7 RNA polymerase promoter-operably linked to an open reading frameencoding an IF. The mmRNA synthesis reaction is carried in the presenceof a mixture of modified and unmodified nucleotides. In some embodimentsmodified nucleotides included in the in vitro synthesis of mmRNAs arepseudo-uridine and 5-methyl-cytosine. A key step in cellular mRNAprocessing is the addition of a 5′ cap structure, which is a 5′-5′triphosphate linkage between the 5′ end of the RNA and a guanosinenucleotide. The cap is methylated enzymatically at the N-7 position ofthe guanosine to form mature mCAP. When preparing IF mmRNAs, a 5′ cap istypically added prior to transfection of hPSCs in order to stabilize IFmmRNA and significantly enhance translation. In some embodiments a 4:1mixture of a cap analog to GTP is used in transcription reactions toobtained 5′-capped mmRNAs. In preferred embodiments, the Anti ReverseCap Analog (ARCA), 3′-O-Me-m7G(5′)ppp(5′)G is used to generate IF mmRNAsthat can be efficiently translated in hPSCs. Systems for in vitrosynthesis are commercially available, as exemplified by the mRNAExpress™mRNA Synthesis Kit (System Biosciences, Mountain View, Calif.).

IF mmRNAs can be introduced into hPSCs by any of a number of establishedmethods for transfection of mammalian cells, e.g., electroporation,nucleoporation, or lipofection. In one exemplary embodiment IF mmRNAsare introduced into hPSCs by nucleoporation as follows.

Nucleofection of IF mRNAs into hPSCs is performed using an Amaxa HumanStem Cell Nucleofector® Kit 2. Prior to nucleofection, cells are washedwith PBS and dissociated to a single cell suspension using Accutase®(Invitrogen) and collected in TeSR1™ medium containing 10 μg/ml ROCKinhibitor (Y27632). For one/well reaction 1.5×10⁶−2×10⁶ cells areresuspended in 100 μl of nucleofection reagent containing mmRNA (1.75 μgof both GATA2 and ETV2; 3.5 μg in total), transferred immediately tonucleofection cuvette, and nucleofected using the B-016 program on theAmaxa Nucleofector II. After the procedure, cells are resuspended in 500μl of TeSR1™ medium with ROCK Inhibitor (Y27632) and transferred toMatrigel™ coated six-well plates containing two ml of TeSR1™ media.Cells are then cultured in a regular TeSR1™ medium for the first 24hours followed by a change to growth factor-free TeSR1™ base mediumcontaining SCF (100 ng/ml), TPO (50 ng/ml) and bFGF (20 ng/ml).

In other embodiments, IF proteins s are generated by in vitrotranslation and then transduced into hPSCs. In some cases, proteintransduction method includes contacting cells with a compositioncontaining a carrier agent and at least one purified polypeptidecomprising the amino acid sequence of one of the above-mentioned IFs.Examples of suitable carrier agents and methods for their use include,but are not limited to, commercially available reagents such asChariot™. (Active Motif, Inc., Carlsbad, Calif.) described in U.S. Pat.No. 6,841,535; Bioport®. (Gene Therapy Systems, Inc., San Diego,Calif.), GenomeONE (Cosmo Bio Co., Ltd., Tokyo, Japan), andProteoJuice™. (Novagen, Madison, Wis.), or nanoparticle proteintransduction reagents as described in, e.g., in U.S. Pat. No. 7,964,196.

The protein transduction method may comprise contacting hPSCs with atleast one purified polypeptide comprising the amino acid sequence of oneof the above-mentioned TAs fused to a protein transduction domain (PTD)sequence (IF-PTD fusion polypeptide). The PTD domain may be fused to theamino terminal of an IF sequence; or, the PTD domain may be fused to thecarboxy terminal of an IF sequence. In some cases, the IF-PTD fusionpolypeptide is added to cells as a denatured polypeptide, which mayfacilitate its transport into cells where it is then renatured.Generation of PTD fusion proteins and methods for their use areestablished in the art as described in, e.g., U.S. Pat. Nos. 5,674,980,5,652,122, and 6,881,825. See also, Becker-Hapak et al (2003), CurrProtocols in Cell Biol, John Wiley & Sons, Inc. Exemplary PTD domainamino acid sequences include, but are not limited to, any of thefollowing:

(SEQ ID NO: 8) YGRKKRRQRRR;; (SEQ ID NO: 9) RKKRRQRR (SEQ ID NO: 10)YARAAARQARA;  (SEQ ID NO: 11) THRLPRRRRRR;  and (SEQ ID NO: 12)GGRRARRRRRR.

III. Compositions

Also described herein are compositions useful for carrying out the hPSCdifferentiation methods described above.

In some embodiments disclosed herein are recombinant human pluripotentstem cells (hPSCs) that comprise: (i) one or more exogenous nucleicacids suitable for expression of any of the combinations of IFs listedin Table 1 or Table 2, or functional homologs thereof (ii) exogenouspolypeptides each of which comprises the amino acid sequence of an IF ora functional homolog thereof.

In some embodiments the recombinant human pluripotent stem cells areintegration-free. In some embodiments, where the recombinant hPSCs areintegration-free, the hPSCs contain one or more episomal mammalianexpression vectors, recombinant viral RNAs (e.g., a Sendai virus RNAgenomes), or mmRNAs encoding any of the IF combinations described inTable 1 or Table 2. In other embodiments the recombinant human PSCscomprise exogenous polypeptides comprising the amino acid sequence ofany of the IFs for the combinations listed in Table 1 or Table 2, orfunctional homologs thereof. In some embodiments the recombinant hPSCsare recombinant hiPSCs. In other embodiments the recombinant hPSCs arerecombinant hESCs

In some embodiments, recombinant hPSCs are provided as a cell culturecomposition for generating hemogenic endothelial cells with pan myeloidpotential, where the cell culture composition comprises recombinanthPSCs and a cell culture medium suitable for expansion of hematopoieticcells. In some embodiments, a suitable cell culture medium includesFGF2, SCF, and TPO.

Also disclosed herein is a kit for hemogenic reprogramming, comprising:

(i) one or more isolated nucleic acids comprising an open reading framefor (a) ETV2 or ERG, and GATA1; (b) ETV2 or ERG, and GATA2; (c) ETV2 orERG, and GFI1; or (c) TAL1 and GATA2; or

(ii) one or more recombinant expression viruses (e.g., retroviruses orlentiviruses) suitable for expression, in human pluripotent stem cells,of (a) ETV2 or ERG, and GATA1; (b) ETV2 or ERG, and GATA2; (c) ETV2 orERG, and GFI1; or (c) TAL1 and GATA2.

In some embodiments, the one or more isolated nucleic acids provided inthe kit are DNA expression vectors. In other embodiments, the providednucleic acids are modified mRNAs.

EXAMPLES Example 1 A Screen for Hematopoietic Induction Factors

Human pluripotent stem cells (hPSCs), including embryonic stem cells(hESCs) and induced PSCs (hiPSCs) offer a plentiful source of bloodcells for experimentation and therapeutic purposes. Although significantadvances have been made in hematopoietic differentiation from hPSCs, abetter understanding of key regulators of hematopoietic commitment isrequired to achieve the scalability of blood cells production from hPSCsand enable de novo generation of hematopoietic stem cells (HSCs).

Transcription factors (TFs) have been recognized as critical regulatorsof early embryonic development. They function as key elements of generegulatory network that guide the acquisition of specific propertiesdefining each particular cell type (1). Several TFs have been identifiedas master regulators of hematopoietic development in mouse embryo (2-5).Many of them are also involved in the regulation of endothelialdevelopment reflecting a close developmental link between endothelialand hematopoietic cells (6). In fact, recent studies have demonstratedthat in the embryo, hematopoietic cells, including HSCs, arise fromendothelial cells with blood-forming potential, hemogenic endothelium(HE) (7-9), indicating that blood development proceeds through anendothelial intermediate stage. To unravel the most essential TFsrequired for the induction of the blood program from hPSCs, we performedcomprehensive gain-of-function screens. Using this approach weidentified two groups of TFs capable of inducing the distinct robusthematopoietic programs from PSCs: pan-myeloid (ETV2 and GATA2) anderythro-megakaryocytic (TAL1 and GATA2). Interestingly, both TFcombinations directly induced hemogenic endothelial (HE) cells, whichsubsequently transformed into blood cells. These results stronglyindicate that specification to discrete types of hematopoieticprogenitors begins at the HE stage and is regulated by distincttranscriptional programs. In addition, we also demonstrated the abilityof modified mRNA (mmRNA) encoding TFs to induce a hematopoietic programin hPSCs without the risk of genomic modifications.

Materials and Methods Cloning of Selected Genes and Virus Production

Open Reading Frames (ORFs) of selected genes were amplified from cDNA ofH1 hESCs differentiated in co-culture with OP9, or from full-lengthcDNAs clones obtained from Open BioSystems and Gene Copoeia Inc. Aftersequence verification, ORFs were subcloned into pSIN/EF1α-IRES-Purolentiviral expression vector. Virus production was carried out bycalcium phosphate transfection of 293T cells. Packaged lentiviral unitswere concentrated on gas-sterilized Centricon Plus-70 or Amicon Ultra-15Centrifugal Filter Units (Millipore) or by ultracentrifugation at 33,000rpm for 2.5 hr and re-suspended in 1%BSA in PBS. Lentiviral stocks weretitrated using puromycin resistant HeLa cells (working concentration ofpuromycin 1 μg/ml), and stored at −80C.

Cell Culture and hPSC Transduction

hESC lines H1(WA01), H9 (WA09) and fibroblast-derived hiPSC (DF-19-9-7Tand DF-4-3-7T) were obtained from WiCell Institute Madison, Wis. Cellwere maintained and expanded in undifferentiated states on mouseembryonic fibroblasts. Prior to lentiviral transduction, hESCs weretransferred on Matrigel™ and grown in feeder free conditions from two tofive passages. After treatment with Accutase® (Invitrogen), hESCs weretransduced in a single cell suspension at concentration 0.68×10⁶cells/ml, in the presence of ROCK Inhibitor (10 m/ml, Stemgent),Polybrene (6 μg/ml, Sigma) and virus (MOI=1-5). Treated cells wereplated on 6 well Matrigel™ coated plates (1 ml/well), and incubated for12 hours. Viral medium was replaced with fresh TeSR1™ and incubated foranother 12 hours. On day 1 after transduction, regular TeSR1™ wasreplaced with TeSR1-growth-factor-free, supplemented with SCF (100ng/ml), TPO (50 ng/ml) and FGF2 (20 ng/ml). Cells were maintained inindicated conditions from three to seven days, depending on theirsurvival and growth, and collected for analysis.

Nucleofection of Human Pluripotent Stem Cells with Modified mRNAs

Nucleofection of H1 hESCs with modified messenger RNAs (mmRNAs) wasperformed using Amaxa Human Stem Cell Nucleofector® Kit 2. Prior tonucleofection, cells were washed with PBS and dissociated to a singlecell suspension using Accutase® (Invitrogen) and collected inTeSR1™medium containing 10 μg/ml Rock inhibitor. For one/well reaction2×10⁶ cells were resuspended in 100 μl nucleofection reagent containingmmRNA (1.75 μg of both GATA2 and ETV2; 3.5 μg in total), transferredimmediately to nucleofection cuvette and nucleofected using the B-016program on the Amaxa Nucleofector II. After the procedure, cells wereresuspended in 500 μl of TeSR1™medium with Rock Inhibitor and transferto Matrigel™ coated 6-well plates containing 2 ml of TeSR1™media. Cellswere kept in a regular TeSR1™medium for the first 24 hours followed bythe change to differentiation medium containing SCF (100 ng/ml), TPO (50ng/ml) and bFGF (20 ng/ml).

Immunostaining Procedures

Expression of cell-surface proteins was assessed by routine flowcytometry protocol (FACSCalibur, BD Biosciences). For intracellularstaining by FACS, cells were fixed for 10 minutes at 37° C. in Cytofixbuffer (BD Biosciences), followed by permeabilization for 30 minutes onice in Perm Buffer III (BD Biosciences). After washing, cells werestained at 40° C. for 2 hours with fluorescence-conjugated antibodies.For detection of protein expression and cellular localization byimmunofluorescence, cells were fixed with 4% paraformaldehyde on cultureplates, permeabilized with 0.01% of Triton X-100, and stained overnightat 40° C. with primary antibodies, followed by staining with thesecondary fluorochrome-labeled antibodies. Intranuclear staining ofpluripotency markers was performed by permeabilization with ice-cold0.2% Triton X-100 in PBS. All antibodies used in this study are listedin Table 4.

Hematopoietic Colony-forming Assay

Hematopoietic clonogenic assays were performed using serum-containingmethylcellulose medium (Meth° Cult) supplemented with SCF, G-CSF,GM-CSF, IL3, IL6, and EPO (Stem Cell Technologies) according to themanufacture's protocol. Wright staining was used to evaluate themorphology of cells within colonies.

Endothelial Assays

Endothelial differentiation was assessed as previously described (10).On day 7 post-transduction, TF-induced cells were placed onfibronectin-coated 6-well plates (hFibronectin, BD) supplemented withcomplete Endothelial Cell Medium ECM (ScienCell). For AcLDL uptakeassay, cells growing in monolayer were incubated with 10 μm/ml ofAlexa-594- or Alexa-488-conjugated AcLDL (Invitrogen, cat.# L-35353 andL-23380 correspondingly) for four hours at 37° C. followed byfluorescent microscopy or flow cytometry analysis. For vascular tubeformation, 2×10⁴ cells were resuspended in ECM medium supplemented withVEGF 40 ng/ml and plated on a solidified Matrigel™-coated 96-well plate.Cells were incubated at 37° C., 5% CO₂ for 18-24 hours when tubeformation was observed.

Evaluation of Hemogenic Potential of Transduced Cells at EndothelialStage

Cells transduced with recombinant viruses for TF expression werecollected at day 3 post-transduction and labeled with VE-cadherin, CD43,and CD73 antibodies. Individual VE-cadherin⁺ CD43⁻CD73⁻ cells were thendeposited into 96-well plates on an OP9 monolayer using FACSAria™ cellsorter, cultured for two weeks and analyzed for CD43 and VE-cadherinexpression by immunostaining.

Quantitative RT-PCR/PCR

RNA isolation was carried out with RNeasy Micro Kit (Qiagen). RNAconcentration and quality was evaluated by nano-drop followed by cDNAsynthesis using AdvantageRT-for-PCR Kit (Clontech). qPCR was performedusing SYBR® Advantage® qPCR Premix (Clontech).

Genomic DNA was isolated using NucleoSpin Tissue XS kit(Macherey-Nagel), and PCR was carried out with Tag 2× MasterMix (NewEngland BioLabs Inc).

RNA-seq Analysis

Total RNA was isolated using RNeasy Micro Kit (Qiagen, cat#74004).Treatment with DNaseI was performed on the column according to themanufacture's protocol. Purity and integrity of RNA was estimated by thecapillary electrophoresis on the Bioanalyzer 2100 (AgilentTechnologies). PolyA+ RNAs were amplified using a modified T7amplification method as previously described (Sengupta at al., 2010).cDNA samples were quantified with the Qubit Fluorometer (Invitrogen) andsequenced on the Illumina Genome Analyzer IIx.

Time-lapse Microscopy

To capture the endothelial-hematopoietic transition, the time-lapsemovies were recorded using Nikon Eclipse Ti-E configured with an A1Rconfocal system and motorized stage (Nikon Instruments Inc. Melville,N.Y.). Cell culture surfaces were washed thoroughly to remove debris,and VE-cadherin-FITC and CD43-PE antibodies were added to a finalconcentration of 100 ng/ml. Movies were made on day 2.5post-transduction for GATA2/TAL1/LM02-induced cells and on day 4 forGATA2/ETV2-induced cells. Images were acquired using Nikon Elements(NIS-element C) imaging software for every 5 minutes with CFI Plan FluorDLL 20× NA 05 WD 2.1 MM objective (Nikon Instruments Inc. Melville,N.Y.). To convert time-lapse serial images to movies, the Quick-timemovies and ImageJ (NIMH, Bethesda, Md.) software were applied.

Results Selection of Candidate Genes and Screening System Design

To induce the hematopoietic program in hPSCs, we first assembled a listof candidate transcriptional regulators involved in mesodermal andangiohematopoietic specification and HSC development through aliterature review. To prioritize the list of genes for screening, weused molecular profiling data obtained from the analysis of geneexpression of hESC-derived mesodermal and vascular progenitors with andwithout hematopoietic potential (10,11). Based on this data we selected27 genes (Table 3 and FIG. 6a ).

TABLE 3 List of Candidate Transcription Factors (“Induction Factors”)TRANSCRIPTION FACTOR SEQUENCE (human) 1 CBFB NM_001755.2 Core-bindingfactor subunit beta (CBF-beta) 2 CDX2 NM_001265.4 Caudal type homeobox 23 CEBAa BC160133.1 CCAAT/enhancer binding protein (C/EBP), alpha 4 EGR1NM_001964.2 Early growth response 1 5 ERG NM_001243428.1 v-etserythroblastosis virus E26 (SEQ ID NO: 3) oncogene homolog (avian) ETS-related gene; transcriptional regulator ERG 6 ETV2 NM_014209.2 ETStranslocation variant 2 (SEQ ID NO: 1) 7 ETV6 NM_001987.4 ets variant 68 FOXF1 NM_001451.2 Forkhead box F1 9 FOXC2 BC113439.1 Forkhead box C2(MFH-1, mesenchyme forkhead 1) 10 FLI1 NM_002017.3 Friend leukemia virusintegration 1 11 GATA1 NM_002049.3 GATA binding protein 1, globin (SEQID NO: 2) transcription factor 1 12 GATA2 BC051342.1 GATA bindingprotein 2, endothelial (SEQ ID NO: 4) transcription factor GATA-2 13GATA3 BC003070.2 GATA binding protein 3 trans-acting, T-cell-specifictranscription factor GATA-3 14 GFI1 BC032751.1 Growth factor independent1 (SEQ ID NO: 5) transcription repressor 15 HAND1 NM_004821.2 Heart andneural crest derivatives expressed 1 16 HES1 NM_005524.3 Hairy andenhancer of split 1, (Drosophila) 17 HHEX NM_002729.4 Hematopoieticallyexpressed homeobox 18 LMO2 NM_001142315.1 LIM domain only 2(rhombotin-like 1) (SEQ ID NO: 7) 19 LYL1 NM_005583.4 Lymphoblasticleukemia derived sequence 1 20 MYB BC064955.1 v-myb myeloblastosis viraloncogene homolog (avian) 21 NAB2 BC065931.1 NGFI-A binding protein 2(EGR1 binding protein 2) 22 NFE2 BC005044.1 Nuclear factor(erythroid-derived 2), 45 kDa 23 RUNX1 isoform RUNX1A NM_001122607.1Runt-related transcription factor 1 (RUNX1) transcript variant 3 Acutemyeloid leukemia 1 protein isoform a 24 RUNX1 isoform RUNX1BNM_001001890.2 Runt-related transcription factor 1 (RUNX1) transcriptvariant 2 Acute myeloid leukemia 1 protein isoform b 25 RUNX1 isoformRUNX1C NM_001754.4 Runt-related transcription factor 1 (RUNX1)transcript variant 1 Acute myeloid leukemia 1 protein isoform c 26 SPI1NM_003120.2 Spleen focus forming virus (SFFV) proviral integrationoncogene spi1, PU-box binding protein (PU.1) 27 TAL1/SCL NM 003189.2T-cell acute lymphocytic leukemia 1 (SEQ ID NO: 6)

We assumed that the ideal hPSC-based system for a gain-of-functionscreen for hematopoiesis-inductive factors should meet two majorrequirements: (1) support the maintenance of untransduced hESCs orEGFP-transduced hPSCs in an undifferentiated state, (2) allow expansionof induced hematopoietic cells generated from hPSCs expressing asuitable combination of genes from our selected set of 27 genes. Wefound that these conditions were met by maintaining hPSCs as a monolayeron Matrigel™ in a serum-free TESR™1 medium supplemented with FGF2 andSCF and TPO hematopoietic cytokines. As shown in FIG. 1b -1 e, in theseconditions untransduced hESCs or EGFP-transduced hESCs remained visiblyundifferentiated, retaining their morphology, cell surface markers andgene expression profile, while, in the case of some of the candidategenes, transduced hESCs yielded a differentiated phenotype, as describedbelow.

Single Factor Screening Identified ETV2 and ERG as TFs Sufficient forDirect Induction of Endothelium from hESCs

To test the functional capacity of individual genes, we analyzed theireffect on morphology, and expression of various mesodermal, endothelial,and hematopoietic markers by flow cytometry 7 days after transduction:APLNR and KDR (mesodermal), VE-cadherin, CD34 CD31 and CD73(endothelial), and CD43 and CD45 (hematopoietic). Morphologic evaluationof cultures revealed three types of outcomes of TF overexpression: (1) achange in morphology, (2) no apparent change in morphology, and (3) celldeath (Table 4 and FIG. 7).

TABLE 4 Differentiation Effects of Transcription Factors (InductionFactor Candidates) Change Cell Day of Markers by FACS* Factor morphologyDeath* Collection APLNR KDR VEC CD31 CD73 CD34 CD43 CD45 1 CBFB No No d5− − − − − − − − 2 CDX2 No No d5 − − − − − − − − 3 CEBPA No No d5 − − − −− − − − 4 EGR1 Yes Yes d5 − − − − − − − − 5 ERG Yes No d5 − − ++++ +++++++ ++ − − 6 ETV2 Yes No d7 + ++++ ++++ ++++ ++++ ++++ + + 7 ETV6 Yes Nod5 − − − − − − − − 8 FOXF1 Yes Yes d3 − − − − ++ + − − 9 FOXC2 No Yes d3− − − − − − − − 10 FLI1 Yes No d3 − − − ++ − − − − 11 GATA1 Yes Nod5 + + − − + ++ + − 12 GATA2 Yes No d7 ++ ++ + + − ++ + − 13 GATA3 YesNo d7 + + − − − ++ − − 14 GFI1 Yes No d5 − − − − − − − − 15 HAND1 Yes Nod5 − − + + ++ − − − 16 HES1 No No d5 − − − − − − − − 17 HHEX Yes No d5 −− − − − − − − 18 LMO2 No No d5 − − − − − − − − 19 LYL1 No No d5 − − − −− − − − 20 MYB No No d5 − − − − − − − − 21 NAB2 No Yes d5 − − − − − − −− 22 NFE2 No No d5 − − − − − − − − 23 SPI.1 Yes Yes d3 − − − − − − − +24 RUNX1A No Yes d3 + − + − + − − − 25 RUNX1B Yes Yes d3 + − + − + − − −26 RUNX1C Yes Yes d3 + − + − + − − − 27 SCL/ No No d5 − − − − − − − −TAL1 Expression Symbol Levels Positive cells (%) − Negative 0-1 + Low2-5 ++ Moderate  5-20 +++ High 20-50 ++++ Very high >50

Although in many cases morphologic changes were non-specific, we noticedthat ETV2 and ERG induced the formation of cells with typicalendothelial morphology. Immunofluorescent and functional analysesrevealed that ETV2 and ERG-induced cells expressed VE-cadherin, CD31,CD34, TEK and KDR endothelial markers, showed AcLDL uptake, and formedvascular tubes in response to VEGF, consistent with endothelial natureof induced cells (FIGS. 1e-1g ). Gene expression analysis revealed thatETV2 or ERG alone are sufficient to induce expression of almost theentire set of genes required for angiohematopoietic development, andgenes typically expressed in endothelial cells (FIG. 2a ). However, theyhad little effect on expression of pluripotency genes. None of theselected genes were able to induce formation of round CD43⁺ blood cells,though weak expression of CD43 by a very few epithelioid cells was notedfollowing the transduction of cells with ETV2, GATA1 or GATA2 (Table 4).Although FOXF1 and HAND1 TFs are shown to be important for lateralplate/extraembryonic mesoderm development in mouse studies (12, 13), wefound that they did not upregulate expression of APLNR or KDRpan-mesodermal markers or genes known to be expressed in lateral platemesoderm. In contrast, we noticed that GATA2 overexpression by itself isa powerful activator of APLNR and KDR expression and repressor ofESC-specific genes (FIGS. 2a and 2b ). Although GATA1 and GATA3 inducedexpression of many endothelial genes similar to GATA2, they also inducedexpression of primitive streak genes, but had little effect onexpression of ESC-specific genes (FIG. 2b ). Pearson correlationanalysis of global gene expression revealed that ETV2, ERG, GATA1,GATA2, GATA3, HHEX, CEBPA, and EGR1 caused the most dramatic changes ingene expression,while LMO2, a transcriptional cofactor that which lacksDNA binding activity, and several DNA binding molecules, including,HES1, TAL1, LYL1 and CBFB, had minimal effect on gene expression inhESCs.

Example 2 Overexpression of ETV2 and GATA2 is Sufficient to InducePan-myeloid Hematopoiesis from hESCs through Hemogenic Endothelium Stage

It is generally accepted that blood formation in the embryo proceedsthrough hemogenic endothelial intermediates. Therefore as a next step wedecided to test whether the addition of known hematopoietic factors toETV2 or ERG endothelium-inductive factors would be sufficient togenerate endothelium with hemogenic potential. Given well-establishedrole of GATA2 and GATA1 factors in hematopoietic development and ourobservation that these factors induce expression of endothelial andhematopoietic genes (FIG. 2a ), we selected these TFs as a first choice.In fact, transduction of hESCs with ETV2 and GATA2 led to formation ofround CD43 positive blood cells with robust erythroid and myeloid CFCpotential (FIGS. 3a and 3 b and FIG. 8a ). Cells collected fromclonogenic cultures of ETV2/GATA2 transduced hESCs robustly proliferatedin serum-containing medium with cytokines. Flow cytometric analysis ofexpansion cultures revealed all types of myeloid cells, includingCD34⁺CD117⁺ primitive progenitors, CD163⁺ macrophages, CD66b⁺granulocytes, CD41a⁺ megakaryocytic and CD235a⁺ erythroid cellsindicating that GATA2 and ETV2 induce pan-myeloid hematopoiesis fromhESCs (FIG. 3c ). GATA1 in combination with ETV2 induced a similarspectrum of hematopoietic colonies, though we noticed an increase in thenumber of erythroid colonies with GATA1. We also noted that stronginduction of CD43⁺ blood cells could be achieved by co-transfectinghESCs with ETV2 and GFI1 (FIG. 8a ). However, these cells demonstrated avery limited erythroid potential and formed mostly granulocytic coloniesin clonogenic medium (not shown). Transfection of cells with ETV2 andGATA3, TALI, or LMO2 induced very few CD43⁺ cells and much lesshematopoietic CFCs as compared with ETV2/GATA2 or GATA1 combination(FIG. 3d and FIG. 8a ). The addition of other factors on the top of theETV2/GATA2 combination did not change substantially the spectrum ofhematopoietic programming, although incorporation of erythroid factorsTAL1 and LMO2 slightly facilitated the development of erythroid (E)colonies, while GFI1 and CEBPA increased frequency of myeloidprogenitors (FIG. 8a-e ). We also found that the hematopoietic programcan be induced by co-transfecting ERG with GATA2 or GFI1 (FIG. 8e ). Thenumber of CFCs induced by these combinations however, was substantiallylower compared to GATA2 or GFI1 combined with ETV2.

These observations indicate that co-expression of endothelial factorssuch as ETV2 or ERG with various GATA2, GATA1, TAL1, or GFI1 TFs leadsto induction of hematopoietic program in hESCs with different efficiencyand spectrum of clonogenic activity. Because GATA2 and ETV2 combinationinduced the most robust multi-lineage hematopoiesis, we concluded thatthese factors are most critical for induction of pan-myeloidhematopoietic program in hESCs.

Gene expression profiling revealed that the combination of ETV2 withGATA2 or GATA1 was sufficient to activate almost the entire spectrum ofgenes essential for hematopoiesis, including endogenous ETV2 and GATA2,TAL1, LMO2, RUNX1, LYL1, and GFI1 among others (FIG. 2a ).Interestingly, following activation of endogenous GATA2 and ETV2 genes,expression of exogenous genes in induced blood cells was dramaticallydownregulated and was hardly detected by PCR FIGS. 6f and 6g ),suggesting that these factors may induce an autoregulatory loop tomaintain their expression.

Kinetic analysis of blood formation by ETV2 and GATA2 transduced cellsrevealed that hematopoietic development from hESCs proceeds through theendothelial stage. Three days after ETV2 and GATA2 transfection, hESCsacquired typical endothelial morphology and phenotypic features similarto ETV2 transduced hESCs (FIGS. 4a and 4b ). However, in contrast toETV2 alone, endothelial cells induced after 3 days of GATA2 and ETV2transduction expressed CD226 and lacked CD73 (FIG. 4c ), i.e. displayedphenotypic features typical of hemogenic endothelium (10). Within thenext two days we observed a transition of endothelial cells into roundCD43⁺ hematopoietic cells, thereby indicating that ETV2 and GATA2overexpression directly induces formation of endothelial cells withhemogenic properties which subsequently gave rise to blood cells (FIG.4b ). When VE-cadherin⁺ cells were collected from ETV2/GATA2 transducedcultures prior to detection of CD43 expression (day 3) and cultured onOP9, they generated colonies of CD43⁺ hematopoietic cells withmultilineage CFC potential (FIG. 9a-c ), indicating thatVE-cadherin⁺CD43⁻ cells induced by ETV2/GATA2 have functional potentialsimilar to hemogenic endothelium generated from hESC by differentiationon OP9 (10).

Example 3 TAL1 and GATA2 Induce Hematopoietic Program Mostly Restrictedto Erythromegakaryocytic Cells

Although the basic helix-loop-helix TF TAL1 is a well-known keyregulator of hematopoiesis and vasculogenesis (14, 15), overexpressionof TAL1 alone was not able to induce formation of blood cells fromhESCs. When added to ETV2, TAL1 induced only a few hematopoieticcolonies (FIG. 3d ). Cotransfection of TAL1 with GATA2 or GATA1 geneshowever, induced the formation of VE-cadherin⁺ endothelial and CD43⁺hematopoietic cells (FIG. 3e ) similar to ETV2/GATA2 combination, but incontrast, hematopoiesis in TAL1/GATA2 or GATA1 transduced cultures waspredominantly restricted to erythroid and megakaryocytic cells with fewmacrophages (FIGS. 3f and 3h ). Interestingly, the formation of CD43⁺round blood cells in cultures was preceded by upregulation ofVE-cadherin expression in transformed cells (FIGS. 4a and 4c ),indicating that CD43⁺ cells generated with these two factors, similar toETV2 and GATA2 transduced cells, arose from endothelial cells throughendothelial-hematopoietic transition. Endothelial cells induced by TAL1and GATA2 on day 3 of culture had phenotypic and functional features ofhemogenic endothelium, i.e. they expressed CD226, lacked CD73 (FIG. 4c), and were capable of growing blood after culture on OP9 (FIGS. 9a and9b ). Addition of LMO2 to the TALI/GATA2 combination dramaticallyincreased hematopoiesis, without significant changes in the spectrum ofhematopoietic colonies (FIGS. 3h and 3g ). When transcriptional cofactorLMO2 was added to TAL1 and GATA2, we observed rapid transition of hESCsinto round CD43⁺ VE-cadherin^(+/−) hematopoietic cells without clearlyidentifiable preceding endothelial stage.

The addition of other factors, including SPI1, and MYB factors that arecritical for definitive hematopoiesis, had no significant effect onTALI/GATA2-induced blood formation and was not able to shifthematopoiesis towards myelomonocytic lineage of cells (FIG. 10b-d ).When cells were transfected with a set of seven genes including TALI,GATA2, LMO2, RUNX1b, RUNX1c, MYB and SPI1, we observed the formation ofnumerous very large red and white colonies. Cells within these coloniespredominantly expressed the erythroid marker CD235a and failed toproduce a significant number of myelomonocytic cells (FIG. 10e ).

GATA2/TAL1/LMO2 transduced cells collected from clonogenic culturesrobustly expanded in serum-free medium with cytokines and generatedalmost exclusively CD235a⁺ erythroid and CD41a⁺ megakaryocytic cells(FIG. 3h ), confirming the restricted differentiation potential of cellsgenerated from hESCs using these TFs.

Example 4 Induction of Hematopoietic Program in hiPSCs and by UsingModified mRNA (mmRNA)

To determine whether the identified sets of transcriptional regulatorswere capable of inducing the hematopoietic program in hPSCs other thanH1 hESCs, we overexpressed ETV2/GATA2 or TAL 1/GATA2/LMO2 in twofibroblast-derived iPSCs. As shown in FIG. 11, hematopoiesis inducedusing these combinations in hiPSCs was similar to what we observed withH1 hESCs. i.e. ETV2 and GATA2 induced pan-myeloid hematopoiesis, whilethe TAL1/GATA2/LMO2 combination induced predominantly the erythroid andmegakaryocytic cells. Pan-myeloid program in hESCs was successfullyinduced by mmRNA indicating that short exposure to TFs is sufficient forthe induction of the hematopoietic program (FIG. 5).

Using a gain-of-function genetic screen we identified ETV2 and GATA2 asthe most critical TFs required for induction of hemogenic endotheliumwith pan-myeloid potential from hESCs. ETV2 and ERG are ETS family ofTFs which play critical roles in endothelial development (16).Gain-of-function experiments in Xenopus and zebrafish embryos havedemonstrated that ERG and ETV2 are able to induce ectopic endothelialdifferentiation (17-19). ETV2 is also required for HSC development fromhemogenic endothelium and the maintenance of adult HSCs (20, 21). Wefound that ectopic expression of ETV2 and ERG in undifferentiated hESCsupregulated expression of genes associated with angiohematopoieticdevelopment and typical endothelial genes resulting in the formation ofendothelial cells. Although overexpression of ETV2 alone inducedexpression of endogenous FLI1, GATA2, and TAL1, genes which form thecore of a gene regulatory network in developing HSCs (22), ETV2-inducedendothelium was lacking significant blood-forming activity. Theoverexpression of GATA2 or GATA1 in addition to ETV2 was required toachieve induction of hemogenic endothelial cells and the formation ofmultipotential hematopoietic progenitors. These findings indicated thatETV2 and GATA2 act at the top of the transcriptional network driving theendothelial and myeloid development from hESCs.

Mouse studies have demonstrated that Tall controls the expression ofseveral important hematopoietic regulators, including Runx1, Erg, Gfi1b, and Gata2 among others (23). Tal1 is considered a key component ofthe regulatory network controlling HSC specification (22). However, TAL1overexpression in hESCs induced only minimal changes in the geneexpression profile indicating that TAL1 target genes in undifferentiatedcells may not have open chromatin structure for access by TAL1 Thecotransfection of TAL1 with GATA1 or GATA2 TFs was sufficient to inducehemogenic endothelium, which in contrast to ETV2/GATA2 orETV2/GATA1-induced endothelium had restricted erythromegakaryocytic andmacrophage potential. While hematopoiesis induced by TAL1 and GATA1 orGATA2 TFs was relatively weak, additional transduction of cells withLMO2 transcriptional cofactor led to robust formation of blood cells ofthe erythromegakaryocytic lineage.

Overall, our studies identified two critical pathways leading to theformation of distinct types of hemogenic endothelium and have provided anovel platform to assess the hematopoietic transcriptional program inhPSCs required for HSC induction. Additionally, these studies offer anovel approach to induce efficient production of endothelium and bloodfrom hPSCs by forced expression of transcription factors.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be apparent to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. It should be understoodthat various alternatives to the embodiments of the invention describedherein may be employed in practicing the invention. It is intended thatthe following claims define the scope of the invention and that methodsand structures within the scope of these claims and their equivalents becovered thereby.

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APPENDIX Amino Acid Sequences of IFs (SEQ ID NOs: 1-7)(DNA binding domains are underlined) ETV2 (SEQ ID NO: 1)MDLWNWDEASPQEVPPGNKLAGLEGAKLGFCFPDLALQGDTPTATAETCWKGTSSSLASFPQLDWGSALLHPEVPWGAEPDSQALPWSGDWTDMACTAWDSWSGASQTLGPAPLGPGPIPAAGSEGAAGQNCVPVAGEATSWSRAQAAGSNTSWDCSVGPDGDTYWGSGLGGEPRTDCTISWGGPAGPDCTTSWNPGLHAGGTTSLKRYQSSALTVCSEPSPQSDRASLARCPKTNHRGPIQLWQFLLELLHDGARSSCIRWTGNSREFQLCDPKEVARLWGERKRKPGMNYEKLSRGLRYYYRRDIVRKSGGRKYTYRFGGRVPSLAYPDCAGGGRGAETQ GATA1 (SEQ ID NO: 2)MEFPGLGSLGTSEPLPQFVDPALVSSTPESGVFFPSGPEGLDAAASSTAPSTATAAAAALAYYRDAEAYRHSPVFQVYPLLNCMEGIPGGSPYAGWAYGKTGLYPASTVCPTREDSPPQAVEDLDGKGSTSFLETLKTERLSPDLLTLGPALPSSLPVPNSAYGGPDFSSTFFSPTGSPLNSAAYSSPKLRGTLPLPPCEARECVNCGATATPLWRRDRTGHYLCNACGLYHKMNGQNRPLIRPKKRLIVSKRAGTQCTNCQTTTTTLWRRNASGDPVCNACGLYYKLHQVNRPLTMRKDGIQTRNRKASGKGKKKRGSSLGGTGAAEGPAGGFMVVAGGSGSGNCGEVASGLTLGPPGTAHLYQGLGPVVLSGPVSHLMPFPGPLLGSPTGSFPTGPMP PTTSTTVVAPLSSERG (SEQ ID NO: 3) MIQTVPDPAAHIKEALSVVSEDQSLFECAYGTPHLAKTEMTASSSSDYGQTSKMSPRVPQQDWLSQPPARVTIKMECNPSQVNGSRNSPDECSVAKGGKMVGSPDTVGMNYGSYMEEKHMPPPNMTTNERRVIVPADPTLWSTDHVRQWLEWAVKEYGLPDVNILLFQNIDGKELCKMTKDDFQRLTPSYNADILLSHLHYLRETPLPHLTSDDVDKALQNSPRLMHARNTGGAAFIFPNTSVYPEATQRITTRPDLPYEPPRRSAWTGHGHPTPQSKAAQPSPSTVPKTEDQRPQLDPYQILGPTSSRLANPGSGQIQLWQFLLELLSDSSNSSCITWEGTNGEFKMTDPDEVARRWGERKSKPNMNYDKLSRALRYYYDKNIMTKVHGKRYAYKFDFHGIAQALQPHPPESSLYKYPSDLPYMGSYHAHPQKMNFVAPHPPALPVTSSSFFAAPNPYWNSPTGGIYPNTRLPTSHMPSHLGTYY GATA2 (SEQ ID NO: 4)MEVAPEQPRWMAHPAVLNAQHPDSHHPGLAHNYMEPAQLLPPDEVDVFFNHLDSQGNPYYANPAHARARVSYSPAHARLTGGQMCRPHLLHSPGLPWLDGGKAALSAAAAHHHNPWTVSPFSKTPLHPSAAGGPGGPLSVYPGAGGGSGGGSGSSVASLTPTAAHSGSHLFGFPPTPPKEVSPDPSTTGAASPASSSAGGSAARGEDKDGVKYQVSLTESMKMESGSPLRPGLATMGTQPATHHPIPTYPSYVPAAAHDYSSGLFHPGGFLGGPASSFTPKQRSKARSCSEGRECVNCGATATPLWRRDGTGHYLCNACGLYHKMNGQNRPLIKPKRRLSAARRAGTCCANCQTTTTTLWRRNANGDPVCNACGLYYKLHNVNRPLTMKKEGIQTRNRKMSNKSKKSKKGAECFEELSKCMQEKSSPFSAAALAGHMAPVGHLPPFSHSGHILPTPTPIHPSSSLSFGHPHPSSMVTAMG GFI1 (SEQ ID NO: 5)MPRSFLVKSKKAHSYHQPRSPGPDYSLRLENVPAPSRADSTSNAGGAKAEPRDRLSPESQLTEAPDRASASPDSCEGSVCERSSEFEDFWRPPSPSASPASEKSMCPSLDEAQPFPLPFKPYSWSGLAGSDLRHLVQSYRPCGALERGAGLGLFCEPAPEPGHPAALYGPKRAAGGAGAGAPGSCSAGAGATAGPGLGLYGDFGSAAAGLYERPTAAAGLLYPERGHGLHADKGAGVKVESELLCTRLLLGGGSYKCIKCSKVFSTPHGLEVHVRRSHSGTRPFACEMCGKTFGHAVSLEQHKAVHSQERSFDCKICGKSFKRSSTLSTHLLIHSDTRPYPCQYCGKRFHQKSDMKKHTFIHTGEKPHKCQVCGKAFSQSSNLITHSRKHTGFKPFGCDLCGKGFQRKVDLRRHRETQHGLK TAL1 (SEQ ID NO: 6)MTERPPSEAARSDPQLEGRDAAEASMAPPHLVLLNGVAKETSRAAAAEPPVIELGARGGPGGGPAGGGGAARDLKGRDAATAEARHRVPTTELCRPPGPAPAPAPASVTAELPGDGRMVQLSPPALAAPAAPGRALLYSLSQPLASLGSGFFGEPDAFPMFTTNNRVKRRPSPYEMEITDGPHTKVVRRIFTNSRERWRQQNVNGAFAELRKLIPTHPPDKKLSKNEILRLAMKYINFLAKLLNDQEEEGTQRAKTGKDPVVGAGGGGGGGGGGAPPDDLLQDVLSPNSSCGSSLDGAASPDSYTEEPAPKHTARSLHPAMLPAADGAGPR LMO2 (SEQ ID NO: 7)MSSAIERKSLDPSEEPVDEVLQIPPSLLTCGGCQQNIGDRYFLKAIDQYWHEDCLSCDLCGCRLGEVGRRLYYKLGRKLCRRDYLRLFGQDGLCASCDKRIRAYEMTMRVKDKVYHLECFKCAACQKHFCVGDRYLLINSDIVCEQDIYE WTKINGMI

1-13. (canceled)
 14. A recombinant human pluripotent stem cellcomprising: (i) one or more exogenous nucleic acids suitable forexpression of (a) an ETV2 or ERG protein, and a GATA1 protein, orfunctional homologs thereof; (b) an ETV2 or ERG protein, and a GATA2protein, or functional homologs thereof; or (c) an ETV2 or ERG protein,and a GFI1 protein (ii) exogenous polypeptides comprising the amino acidsequences of any of (a), (b), or (c).
 15. The recombinant humanpluripotent stem cell of claim 14, wherein the exogenous polypeptidescomprise the amino acid sequence of a protein transduction domain. 16.The recombinant human pluripotent stem cell of claim 14, wherein therecombinant human pluripotent stem cell is integration-free.
 17. Therecombinant human pluripotent stem cell of claim 16, wherein the one ormore exogenous nucleic acids are episomal plasmid expression vectors.18. The recombinant human pluripotent stem cell of claim 16, wherein theone or more exogenous nucleic acids are modified mRNAs (mmRNAs).
 19. Acell culture composition for generating human hemogenic endothelialcells with pan-myeloid potential, comprising the recombinant humanpluripotent stem cell of claim 14 and a cell culture medium suitable forexpansion of hematopoietic cells.
 20. The cell culture composition ofclaim 19, wherein the cell culture medium comprises FGF2, SCF, andthrombopoietin. 21-22. (canceled)